This invention relates generally to laminate films and more specifically to laminate films in which individual layers of the laminate are bonded together by means of corona discharge treatment. This invention also relates to an apparatus for producing a multilayer laminate utilizing corona discharge treatment, and a method for producing such a laminate.
The use of corona discharge treatment is well known in connection with preparing polyolefin surfaces for printing inks, and for preparing laminated plastic films.
Thick, oriented polyolefin sheets of for example about 10 mils or greater, have been virtually unknown because of the complexities of orienting materials this thick. While it is possible to use conventional laminating techniques for producing relatively thick polyolefin laminates, such a method is quite costly.
The laminate could be passed through a single stage corona discharge treatment to produce the thick laminate, but it has been observed that treatment level changes as dielectric material builds up in the laminate, and the permissible thickness of the resulting laminate is thereby limited.
Of interest is U.S. Pat. No. 3,171,539 issued to Holbrook et al and showing the treatment of both sides of an irradiated, biaxially oriented polyethylene to ensure bonding when the upper surface of the polyethylene material is contacted with its lower surface.
Also of interest is U.S. Pat. No. 3,575,793 issued to Paisley which discloses the use of a biaxially oriented polypropylene layer which is corona bonded to a cellophane layer which has been coated with saran. Additional outside layers of polymeric material may be laminated or coated to the opposite surfaces of the polypropylene or the cellophane. A bond is formed between the corona treated surface of the polypropylene and the saran coating of the cellophane material.
Of interest is Canadian Patent Specification No. 1041892 issued to Dawes et al and discussing corona treating of facing surfaces of various polymeric materials, including ethylene homopolymers and copolymers, polyamides, and ionomers, and coating these materials with polyvinylidene chloride. In one embodiment, polyhexamethylene adipamide is treated on both of its surfaces with corona discharge and sandwiched by outside layers of ethylene butylene copolymer. All of the materials of this embodiment are blown films.
It is desirable to conveniently produce at relatively low cost, in a continuous fashion, a relatively thick laminate preferably containing one or more component layers of oriented materials, or a laminate in which all the layers are oriented.